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Tracking our Graduates' Project

The College of Medicine, Swansea University is interested in understanding what happens to our graduates when they leave their undergraduate medical training.

I'm not directly involved with this project, but I'm spreading the word.

If you're in your final year of study, or if you started your studies in 2006 and you were a Graduate Entry Medicine student with us please take a look at the Facebook page and you could win a £150 Amazon voucher if you answer a quick survey.

http://www.facebook.com/trackingproject

Week 222: spinal tracts

Ouch - we had some monster neuroanatomy learning outcomes this week. From an anatomy perspective we want students to be aware of the major motor and sensory tracts of the spinal cord, what type of information they carry, where they run (and where they cross over from one side to the other), and what would happen if the spinal cord or brainstem was damaged on just one side in terms of sensory info coming into the brain and muscle control. (It's pretty easy to work out what would happen if the spinal cord or brainstem is completely severed).

I've included a bunch of images, but excuse my scribbles, lack of legends and minimal labels. Some of this is intentional, and some of it suggests I don't have enough time to do this as completely as I would like (I tried to write this when I taught it last year and have had it on my to do list for 12 months!) The images below are of a spinal cord in transverse section with some of the spinal tracts indicated by coloured blobs, and of the brain, midbrain and spinal cord cut in coronal section with the paths of neurones indicated. Yes, really, that's the brain at the top of the diagram. The thick grey is the grey matter of the cerebral (and cerebellar) cortex.

We started with the spinal cord. By now you guys have a pretty good idea of the structure of the spinal cord, and we ran through the white (myelinated neurone axons running up and down the spinal cord) and grey (cell bodies, synapses) matter, the ventral horns (motor neurones go out) and dorsal horns (sensory neurones come in) and talked briefly about the terms "commissure" and "decussation", because the concept of neurones crossing from one side of the body to the other is important.

Tracts within the spinal cord are bundles of neurones running together, usually with a common function and going to or coming from similar places. A fascicle does the same thing. Some areas of the spinal cord are described as columns, such as the dorsal column of the spinal cord's white matter, in which tracts run bunched together.

Some major tracts to be concerned with are the corticospinal tracts (ok, there are lateral and anterior corticospinal tracts so that's two on either side), spinothalamic tracts, dorsal column tracts, and maybe spinocerebellar tracts.

Corticospinal tracts carry motor neurones (and upper motor neurones at that). Upper motor neurones start in the cortex of the brain, namely the motor cortex (or precentral cortex), run through the white matter of the brain to the brainstem, cross over to the other side of the body (decussate) in the pyramidal decussation of the medulla, and then carry on down the spinal cord in the lateral corticospinal tract. At the appropriate level they synapse with a new motor neurone (this would be a lower motor neurone) in the ventral horn of the spinal cord, which carries the action potential out along a spinal nerve and peripheral nerves to the target muscle (and the muscle contracts). The muscle that contracts is on the opposite side of the body to the side of the brain that started this, in the motor cortex.

Well, this happens for around 90% of upper motor neurones. The rest run down the same side of the body as the side of the brain that they started from, in the anterior corticospinal tract. They tend to control muscles of the body wall.

If the spinal cord is damaged on only one side, muscles on the same side as the injury (ipsilateral) will be affected because they decussated up in the medulla. (Damage to upper motor neurones tends to give muscle weakness, increased muscle tone and changes in reflexes.) Theoretically if the brainstem was damaged superior to the pyramidal decussation, effects on muscles on the opposite side to the lesion (contralateral) would be seen.

Dorsal columns and spinothalamic tracts carry sensory neurones up the spinal cord to the thalamus (which filters sensory information before relaying appropriate stuff to the somatosensory cortex).

The dorsal columns contain neurones conveying fine touch, vibration, two-point discrimination and proprioception. Peripheral neurones pass into the dorsal horn of the grey matter, pass into the dorsal column (without synapsing) and stay on the same side of the spinal cord, running superiorly to the brainstem, where they synapse with a second order neurone. This second order neurone crosses to the other side of the body and ascends to the thalamus.

Spinothalamic tracts convey neurones of pain, temperature and crude touch. In these cases peripheral neurones enter the dorsal horn of the grey matter, and may ascend on the edge of the dorsal horn for a couple of spinal segments. There they synapse with a second neurone, which crosses to the other side of the spinal cord and ascends within the spinothalamic tracts through the brainstem to the thalamus.

If we consider injury to the brainstem again, in the case of sensory information the patient would lose senses of pain, touch and proprioception on the opposite side (contralateral) of the body to the lesion (because all of the sensory neurones have crossed to the other side by this level).

If we consider injury to the spinal cord, the patient would lose senses of pain and temperature on the opposite side of the body to the lesion (those neurones crossed sides already) but would lose senses of touch and proprioception on the same side as the lesion (those neurones will not cross sides until they get to the brainstem).

Weird, huh? Have a read about Brown-Sequard syndrome.

Week 219 - blood circulation in the lungs

The Christmas holidays are over and we're back to teaching (and learning). This week we went back to the lungs and added a little detail by looking at the pulmonary vessels in more detail, and adding the bronchial arteries and veins to this. As this is a teaching week about pulmonary embolism this anatomy should be very helpful.

We recapped how the pulmonary trunk comes out of the right ventricle of the heart, carrying deoxygenated blood towards the lungs. It splits into left and right pulmonary arteries, and each one passes into the hilum of a lung. These vessels are big. They're carrying a lot of blood that will be 'processed' (for want of a better word), like we see in the liver and the kidneys. This isn't blood that supplies the lung tissue as such, it is blood that needs some gases removed from it and other gases added to it.

The pulmonary arteries divide as soon as they enter the lung, and if you look at cadaveric lungs you may see this if they have been cut away from the body more deeply. A superior lobar branch passes to the upper lobe of the lung. In the left lung the other branch passes to the lower lobe as an inferior lobar branch. In the right lung a branch descends a little way and splits to give off lobar branches to the middle lobe and the lower lobe.

The lobar branches give off yet smaller branches, known as segmental branches. Branching continues until the pulmonary arteries (or arterioles when they become small) reach the alveoli and form capillary beds at the blood-air barrier, where gaseous exchange occurs.

The interesting thing here is that the branching pattern of the pulmonary arteries matches the branching pattern of the airways. I.e. the pulmonary arteries enter the hila of the lungs with the main bronchi, and the lobar branches pass with lobar bronchi, and segmental branches pass with segmental bronchi. Note that these branches of both structures may also be referred to as secondary (lobar) and tertiary (segmental) branches.

Remember that the bronchi continue as conducting bronchioles, then terminal bronchioles, and then respiratory bronchioles (and then alveoli).

From the capillary beds of the alveoli freshly oxygenated blood passes through small pulmonary venules, which drain into ever larger vessels, until they become the pulmonary veins and leave the lungs, returning this blood to the left side of the heart. Note that the tributaries (i.e. the branching pattern) of the pulmonary veins does not match the branching pattern of the airways. It looks very similar but it is located in a slightly different region.

The bronchi, pulmonary arteries and pulmonary veins are all great big tubes entering and leaving the lung at the hilum. There are some smaller vessels there too: the bronchial arteries and veins. The bronchial vessels help supply nutrients and oxygen to some parts of the lungs. The bronchial arteries also follow the airways, and can be found on the posterior surfaces of the bronchi and their branches. The two left bronchial arteries commonly come directly from the aorta, and the single right bronchial artery often comes from the (thirdish) posterior intercostal artery, but these origins are a little variable.

At the distal ends of these branches there are anastomoses between the bronchial arteries and the pulmonary arteries. In a younger, fitter adult these anastomoses may be helpful in minimising the ischaemic effects of a pulmonary embolus blocking a segmental pulmonary artery. Much of the blood that enters the lungs within the bronchial arteries leaves within the pulmonary veins. This mixing lowers the oxygen content of the blood within the pulmonary veins a little. Bronchial veins carry some blood away, and drain into the azygos vein (on the right) and the hemiazygos vein (on the left).

Consider the branching above. The smallest segment of a lung that is supplied by a segmental bronchus and a segmental branch of a pulmonary artery (and there will be a bronchial artery along side these two) is known as a bronchopulmonary segment.

This is handy to to be aware of. It would be possible to remove a single bronchopulmonary segment (say, because of a cancerous mass of cells) without affecting the function of any other bronchopulmonary segment. All the other parts of the lung would still have their own airway and blood supply. Also, if you know your bronchopulmonary segments and you know where a patient's pulmonary embolus has lodged you can predict which parts of the lung are likely to be affected.

There are 10 bronchopulmonary segments in the right lung, and 8-10 bronchopulmonary segments in the left lung. Try to think of each segment as kind of pyramid shaped, with the base of the pyramid forming the outer surface of the lung and the pointy apex of the pyramid pointing towards the hilum (where the branches come from).

In between the bronchopulmonary segments are thin sheets of connective tissue. In these spaces between the segments the pulmonary veins are found, running towards the hila and out of the lungs.

Below is the diagram I sketched on the Smartboard during teaching. The black lines are the airways, the blue lines with them are the pulmonary arteries, the thin red line with them is the bronchial artery, and the other lines have labels. Amazing stuff. I hope it made sense at the time. Maybe I should draw this again slowly and it might become useful.

Week 107 - kidneys, nearby viscera and the nervous system

On Monday we spent the morning looking at the kidneys and suprarenal (or adrenal) glands. You saw their anatomy and histology in other stations, but with me we had a quick look at the anterior visceral relationships of each kidney, and then talked about the overall scheme and structure of the nervous system.

The kidneys lie between the peritoneum of the abdomen, and the musculature of the posterior abdominal wall. They are surrounded by fat and fascia, but the point I want to make here is that they are retroperitoneal. Some of the gastrointestinal tract and other abdominal organs are also retroperitoneal.

The left kidney is a little lower than the right kidney. The liver takes up a big space on the right hand side of the abdomen. The left kidney has the left suprarenal gland sat upon its superior pole, and the spleen lies a little laterally to this. Part of the stomach is also anterior to the superior pole of the left kidney. Anterior to the upper middle part of the left kidney lies the tail of the pancreas. This is also retroperitoneal. The left colic flexure (also known as the splenic flexure, because the spleen is here) of the large bowel lies anterior to the left kidney, along with loops of small bowel (jejunum).

The right kidney also has a suprarenal gland upon its superior pole, and the liver is here too. The descending duodenum runs along the medial part of the anterior surface of the right kidney, and is also retroperitoneal. Inferiorly the right colic flexure (or hepatic flexure) and loops of small bowel lie anterior to the kidney too.

We built this up using the plastic torso models, and you might want to do this again yourself. Try to imagine where the peritoneum covers the posterior abdominal wall, and where it reflects up to surround abdominal structures.

The nervous system.

At this stage (week 7) it is useful to have an overview of the anatomy of the nervous system. You've been hearing and using lots of terms for various parts in recent weeks, so let's tie it all together to see if you have developed a good feel for the system.

We can divide the nervous system into two parts: somatic and visceral. The word "somatic" is derived from the Greek word, "soma", meaning "the body". It refers to the frame of the body, rather than to the organs. So the somatic part of the nervous system refers to the parts of the body under voluntary control, largely skeletal muscles. When I described this I tried to get you thinking in motor terms only, for simplicity, but the somatic part of the nervous system includes neurones involved in the sensory input that keeps the body in touch with its surroundings, i.e. external sensory input from the skin, sight and sound.

So the visceral part of the nervous system must drive and receive sensory information from everything else. Smooth muscle and cardiac muscle are innervated by nerves of the visceral nervous system, as are the organs (the viscera), and sensory (afferent) fibres conveying noxious stimuli (pain) and other sensory information back to the central nervous system. You might want to call these visceral sensory neurones, "general visceral afferent fibres". This is all under unconscious, or involuntary control, so this part of the nervous system is more often referred to as the autonomic nervous system. The autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems.

Jumping back a bit, we can also divide the nervous system into the central nervous system (the brain and spinal cord) and the peripheral nervous system (nerves coming out of or going back into the central nervous system). The somatic and visceral parts of the nervous system are generally regarded as sub-divisions of the peripheral nervous system, but when you look at the brain and spinal cord anatomically next year you will see that this view is not always as clear cut as you might like it to be. We'll save that for later though. It's good fun.

Back to the autonomic nervous system: the sympathetic part of the nervous system is often described as triggering the "fight or flight response". If a terrifying cardiothoracic surgeon pounces on you with questions about a coronary angiogram that you struggle to understand, your body may respond by reducing blood flow to organs that aren't useful right now, diverting blood flow to skeletal muscles and releasing glucose into the blood from stored glycogen, preparing you to strike the surgeon (probably not the best option) or flee. This is the sympathetic nervous system acting. Interestingly, as you looked at the suprarenal glands this week, the medulla of this gland contains chromaffin cells that are hard wired directly to sympathetic neurones. These neurones instruct the chromaffin cells to release adrenaline into the bloodstream, triggering body-wide fight or flight responses. Sympathetic neurones are found all over the body, as they drive vasoconstriction, including blood vessels of the skin.

The parasympathetic part of the nervous system is often mentioned as being involved with "rest and digest" functions. Sometimes the parasympathetic and sympathetic parts of the nervous system work in opposition, but often they have somewhat separate functions. Bear this in mind when you are trying to understand their effects on structures and systems in different parts of the body. Parasympathetic neurones will increase mucosal gland and salivary gland activity, increase intestinal activity, for example, but it will also slow the heart rate, whereas the sympathetic nervous system increases the heart rate.

We talked a bit about the anatomy of the autonomic nervous system. We looked at how the sympathetic nerves leave the spinal cord and pass to the sympathetic trunk, two chains of ganglia (ganglion = collection of nerve cell bodies) running on either side of the vertebral column from the pelvis to the neck. The trick here is that sympathetic nerves only leave the spinal cord from levels T1 to L2, but once in the sympathetic trunk they can ascend or descend a little way before leaving to reach other parts of the body. Before they leave the sympathetic trunk most neurones will synapse with another neurone in a ganglion. So we call the fibres going into the sympathetic trunk "preganglionic sympathetic neurones", and those leaving (a second neurone that has synapsed with the first and received the action potential) "postganglionic sympathetic neurones".

If sympathetic nerves leave the spinal cord in the middle (thoracic and upper lumbar segments), then parasympathetic nerves leave the central nervous system at either end; that is, some cranial nerves coming out of the brain contain parasympathetic neurones, and spinal nerves of the sacral plexus contain parasympathetic neurones that enter the pelvis. You've seen how parasympathetic innervation gets into the thorax and most of the abdomen: the vagus nerve (cranial nerve X) descends in the neck and runs through both the thorax and the abdomen.

If a (nerve) ganglion is collection of nerve cell bodies (synapsing with incoming axons of other neurones) what is a nerve plexus? A plexus is a collection of nerve fibres running together, crossing over, changing direction, and maybe forming new, larger nerves. There are no synapses here. There's no informational exchange between nerve fibres in a plexus. Think of it more as a collection of cables that are being organised with cable tidies (thanks for that idea, year 1).

On the abdominal aorta we find both ganglia and plexuses. Near the branch of the coeliac trunk from the aorta we find a couple of masses called the coeliac ganglia, and a meshwork of autonomic nerve fibres running away with the major arteries to pass to the viscera of the abdomen. Going back to the kidney, we might say that the kidney receives autonomic nerve fibres from the aorticrenal plexus (see the parts of the name in there). Sympathetic neurones are vasomotor to the afferent and efferent arterioles within the kidney, but it's unclear what role parasympathetic innervation may play here. Regulation of kidney function is predominantly influenced by hormones.

When we look at the abdomen again in future sessions, and when we cover the embryology of the endocrine system, you will start to add to these ideas of autonomic innervation. Hopefully with time you'll build up a picture of the wiring of the gastrointestinal system and related organs.

Week 104: the neck

In anatomy today we jumped between different topics in neck anatomy, filling gaps, as it were. This was probably a tougher session than previous weeks, and it might take you a while to get through this week's learning outcomes. Anatomy will take up a fair amount of your time this year, but it is an important foundation to lay.

We looked at the bones of the neck, and ticked off the manubrium, the 1st rib and the clavicles as marking the inferior border of the neck, and the mandible and inferior parts of the skull as the superior border of the neck. I pointed out the mastoid process, which is part of the temporal bone in the skull. You saw the sternocleidomastoid muscle that was attached here in another anatomy station. We also briefly looked at the hyoid bone, which many of the strap muscles of the neck attach to.

Digging deep looking at illustrations, Google's body browser (you'll need Google's Chrome browser installed for this to work) and plastic models we found the anterior scalene muscle. It has siblings in the middle and posterior scalene muscles, and runs from the transverse processes of the cervical vertebrae (C3 - C6) down to the first rib. This muscle will elevate the ribcage, or flex the neck to either side.

The reason we picked out the anterior scalene muscle is because of other key structures that pass near to it. The brachial plexus and the subclavian artery passes between the anterior and middle scalene muscles, superior to the 1st rib on their way out to the arm. The phrenic nerve lies on the anterior surface of the anterior scalene muscle and then runs down into the thorax. The carotid sheath, containing the internal jugular vein, the common carotid artery and the vagus nerve, runs anterior and medial to the anterior scalene muscle too.

You now have a landmark to relate these important structures to, so you will be able to describe where they run as your knowledge of them develops. This will also help you find them.

We briefly touched on the blood supply to the trachea and upper oesophagus, linking it to the thyroid gland and the thyroid vessels (particularly the inferior thyroid arteries and the plexus of thyroid veins).

Finally we continued on from last week, where we looked at the arch of the aorta and the descending aorta in relation to the lungs. From the aortic arch 3 major blood vessels branch. The first branch is the brachiocephalic trunk. "Brachium" refers to the arm, and "cephalic" refers to the head, so this artery will supply blood to the arm and the head. It crosses across to the right side of the thorax behind the manubrium, and splits into the right subclavian and right common carotid arteries. The next branch from the arch of the aorta is the left common carotid artery (which is in the appropriate spot to be able to ascend directly up the neck because the aorta has already arched over to the left), and the third branch is the left subclavian artery.

Take a look at the torso models in the lab - some have the scalene muscles and blood vessels, some have the scalene muscles and nerves. Take a good look at how the arch of the aorta not only arches to the left, but also arches posteriorly to reach back to the posterior wall of the thorax. This is something that is often missed when just looking at in images in textbooks.

One of our models has a major flaw in the great vessels of the thorax and neck. if you didn't find it on Monday have a good look at all of the torso models and see if you can find it.